What you are conveying is that from the input block the process is split into two sub-processes. The Ack lines can be repeated at top and bottom.

I would normally sketch a diagram in a program such as OpenOffice and then draw them using TikZ for inclusion in documents. It takes me about 5 minutes with OpenOffice and anything from three quarters of an hour (still) to an hour to get them right using TikZ, so I am not posting any code. If you adopt the approach, please post your code.